1. Field of the Invention
The present invention relates to semiconductor storage devices such as a dynamic random access memory (DRAM) which include a redundancy area as an area provided in a memory cell array, and in particular to a semiconductor storage device for which a refresh test of a redundancy area is performed.
Priority is claimed on Japanese Patent Application No. 2007-331550, filed Dec. 25, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
In semiconductor storage devices such as a DRAM, data retained in memory cells is refreshed at a predetermined cycle. A column-address-strobe (CAS) before row-address-strobe (RAS) refresh (hereinafter referred to as “a CBR refresh”) is frequently used as such a refresh.
In a refresh operation in accordance with the CBR refresh, after a CBR refresh command has been input to a semiconductor storage device chip, a refresh address is automatically determined by the semiconductor storage device, a word line corresponding to the refresh address is activated by an X (row) decoder, and a refresh operation is automatically performed in the semiconductor storage device. Moreover, the refresh address is incremented or decremented every time that a CBR refresh command is input, so that different word lines are activated successively and refresh operations of memory cells connected to the respective different word lines are performed successively.
There is also a refresh method in which a refresh operation is performed in response to the input of an Activate command and to the subsequent input of a Precharge command (hereinafter referred to as “Activate command→Precharge command”). In this refresh operation, an Activate command for a memory cell array is input to a semiconductor storage device, and a refresh address is input to the semiconductor storage device at the same time. Subsequently, after a predetermined time tRAS (i.e., a waiting time from when the Activate command has been input until when a Precharge command is input) has elapsed, the Precharge command is input to the semiconductor storage device to precharge bit lines, and then a refresh operation is performed. In accordance with this refresh method, it is necessary to input a refresh address every time an Activate command is input. Moreover, the time required for the refresh operation is determined by the interval between commands input from the outside of the semiconductor storage device (i.e., an input interval between the Activate command and the Precharge command).
The present inventor recognized the following matters.
There are several problems in methods for performing a refresh test of a redundancy area in a semiconductor storage device.
As a first problem, although semiconductor storage devices may be configured to allow inputting CBR refresh commands for a normal area, such semiconductor storage devices are not configured to allow inputting CBR refresh commands for a redundancy area. For this reason, it is impossible to perform CBR refresh operations for the redundancy area, and hence refresh operations in accordance with the sequence of “Activate command→Precharge command” are used instead of the CBR refresh operations.
As a second problem, defective bits detected by a refresh test in accordance with a refresh method for a normal area may be different from those detected by a refresh test in accordance with a refresh method for a redundancy area due to a difference between these refresh methods. This problem is caused by a difference in internal operation timing between refresh operations in accordance with a CBR refresh command and those in accordance with the sequence of “Activate command→Precharge command”. Specifically, in accordance with the CBR refresh command, the refresh operations are realized by automatically performing an activation operation followed by a precharge operation (hereinafter referred to as “Activate→Precharge”) in the chip. In contrast, in accordance with the sequence of “Activate command→Precharge command”, the refresh operations are performed in response to the commands supplied from a tester.
Therefore, it is desirable to avoid a situation in which defective memory cells cannot be detected by a refresh test of a redundancy area, which is caused by a difference in timing between refresh operations in accordance with the CBR refresh command and refresh operations in accordance with the sequence of “Activate command→Precharge command”, by making it possible to input CBR refresh commands for the redundancy area. In other words, it is desirable to detect unstable bits (memory cells) having poor hold characteristics by performing CBR refresh operations for the redundancy area under the same conditions as those when semiconductor storage devices are actually used after the shipment thereof, thereby allowing the replacement of the unstable memory cells with redundancy memory cells. It is also desirable to solve the problem in that when CBR refresh operations for memory cells in the redundancy area are performed, the time required for the refresh operations increases in proportion to the number of the word lines in the redundancy area (hereinafter may be referred to as “redundancy word lines”).
Meanwhile, there is a semiconductor storage device which performs CBR refresh operations for a redundancy area (see Japanese Unexamined Patent Application, First Publication No. 2002-124096 (hereinafter referred to as “Patent Document 1”)). The Patent Document 1 aims at providing a semiconductor storage device which is capable of performing CBR refresh tests in the same manner as those for normal memory cells by inputting CBR commands of the same number of times as the number of the redundancy word lines and thereby ensuring the reliability of the redundancy word lines. To this end, the Patent Document 1 is provided with a redundancy CBR refresh counter. When performing a refresh test for redundancy memory cells, the redundancy CBR refresh counter is activated every time a control signal is input, the number of CBR commands which have been input is counted by the redundancy CBR refresh counter, and the counted value is output to an X address buffer as a redundancy counter signal. The X address buffer switches between an address counter signal output from a CBR refresh counter and the redundancy counter signal output from the redundancy CBR refresh counter so as to correspond to a case of performing a refresh test of memory cells and a case of performing a refresh test of redundancy memory cells, and outputs the switched counter signal as a counter signal used for generating an internal X address (XA0-XA11).
In this way, the semiconductor storage device of the Patent Document 1 switches between the address counter signal output from the CBR refresh counter and the redundancy counter signal output from the redundancy CBR refresh counter so as to correspond to the refresh test of the memory cells and the refresh test of the redundancy memory cells, and outputs the switched counter signal. As a result, it is impossible to test both the memory cells and the redundancy memory cells simultaneously, and hence it is necessary to test the memory cells and the redundancy memory cells separately, resulting in an increase in time required for the refresh test. There is also a problem in that control becomes complex because it is necessary to switch between the address counter signal output from the CBR refresh counter and the redundancy counter signal output from the redundancy CBR refresh counter, and to output the switched counter signal.